Polyolefins such as polyethylene and polypropylene are lightweight and inexpensive, and have excellent physical properties and processability. On the other hand, their high chemical stability is an obstacle in achieving high functions such as printability, coating property, adhesion, heat resistance, impact resistance, hydrophilicity, stimulation responsiveness, and compatibility with a polymer having polarity. This defect of polyolefins is widely compensated for by copolymerizing ethylene and a polar group-containing monomer such as vinyl acetate or methacrylic acid ester by a high-pressure radical polymerization method, or by grafting a polyolefin with a polar group-containing monomer such as maleic anhydride in the presence of a peroxide. In addition, Japanese Patent Application Laid-Open (JP-A) No. 8-109218 discloses a method in which a polyolefin is obtained by polymerization and an end of the polyolefin is modified. JP-A No. 2002-145944 discloses a method of copolymerizing an olefin and a polar group-containing monomer. By these methods, various polyolefins having polar groups are obtained. However, the polyolefins obtained by these methods generally contain only a small amount of polar groups. Moreover, the polar group-containing monomers are separate from one another in the olefin chain. Even if the polar group-containing monomers form a chain, there will most often be a few such chains. Accordingly, coating property, adhesion, and compatibility with a polar resin are not sufficient in some cases.
A solution to such problems will be hybrid polymers composed of a so-called polar polymer segment in which polar group-containing polymers are linked, and a polyolefin segment.
As a method of producing such a polymer, for example, WO 98/02472 discloses a method of preparing block polymers, in which an alkylboron-containing polyolefin is treated to convert the boron-containing group into a peroxide, and a monomer such as methyl methacrylate is caused to undergo radical polymerization.
In addition, JP-A No. 2004-131620 by the present applicant discloses a method in which a polyolefin obtained by copolymerization of an olefin and a polar group-containing monomer is treated to convert the polar group into a radical polymerization initiator, and a polar group-containing monomer such as methyl methacrylate is allowed to undergo radical polymerization.
Among the above methods, the former method entails use of an alkylboron-containing polyolefin which is prepared by introducing a boron compound in a polyolefin by modifying an unsaturated bond in the polyolefin with a special boron compound or by copolymerizing an olefin and a boron-containing olefin. However, the boron compounds are expensive and the step for modification or copolymerization adds costs, causing industrial disadvantages. Further, the peroxide used as the polymerization initiating site is chemically unstable and often causes nonuniform polymerization. Consequently, it is difficult to regulate the polymerization such that the resulting polar polymer segment has a desired polymerization degree and molecular weight. On the other hand, in the method in which the polyolefin obtained by copolymerization of an olefin and a polar group-containing monomer is treated to convert the polar group into a radical polymerization initiator, the radical polymerization proceeds in a relatively controlled manner such as so-called atom transfer radical polymerization or nitroxide-mediated radical polymerization. Therefore, the method is free of the problems of nonuniform polymerization and difficult control of polymerization degree and molecular weight of the resulting polar polymer. However, the preparation of the olefin/polar group-containing monomer copolymer as raw material for the radical polymerization initiator involves a special metallocene catalyst or addition of a large amount of alkylaluminum. Moreover, because of the presence of the polar group containing monomer, the polymerization efficiency is not high. Therefore, there is a need for a simpler method for converting a polyolefin into a radical polymerization initiator. In the methods described above, the polyolefin segment and the polar polymer segment are connected with a bonding group containing a heteroatom such as an ester linkage or an ether linkage. Therefore, the copolymers are susceptible to hydrolysis.
Further, CN 1165831 discloses a method of producing a grafted copolymer of a rubber and a radically polymerizable monomer. In the method, a raw material is a double bond-containing rubber, such as an ethylene-propylene-diene rubber and a styrene butadiene rubber, obtained by copolymerizing a diene compound. The raw material contains an allyl, which is halogenated. The halogenated rubber is used as a radical polymerization initiator for copolymerization with styrene or methylmethacrylate. According to this method, a hybrid polymer may be produced using a polyolefin-based rubber originally having a double bond in the molecule. However, the method is applicable only to diene copolymers as raw materials, and polyolefins which are commonly produced in the industry such as polyethylene and polypropylene cannot be used for the production of hybrid polymers.
For industrial use, the polymers are preferably produced in the form of powder having good properties. However, the polymers which are a polymer solution or molten in the polymerization reaction contain the polyolefin and polar polymer which are essentially different in properties. Therefore, it is extremely difficult to recover the polymers by an industrial useful technique such as reslurrying or crystallization. In addition, deashing is necessary in order to remove low-molecular residues such as a transition metal catalyst used in the radical polymerization, causing industrial disadvantages.    Patent Document 1: JP-A No. 8-109218    Patent Document 2: JP-A No. 2002-145944    Patent Document 3: WO 98/02472    Patent Document 4: JP-A No. 2004-131620    Patent Document 5: CN 1165831